Resolved -
Between December 20, 2025, 16:00 UTC and 20:00 UTC and again December 21, 2025 00:39 UTC and 02:35 UTC, several clients with rerouting enabled, experienced Non-Delivery Reports (NDRs) or were unable to send emails due to a Spamhaus (Microsoft’s default spam service provider) blocklisting our US IP address.
Our US IP address (US Data Center) was temporarily blocked by Spamhaus, which is used by Microsoft as its default spam filtering service. During this period, one of our mail rerouting servers became overloaded due to a high volume of queued messages, reaching its processing limit. This led to additional delivery delays and Non-Delivery Reports (NDRs) for some clients.
Our Spamhous monitoring system triggered an alarm, allowing us to quickly identify the issue and request immediate removal from the blocklist. The IP was successfully removed from the Spamhaus blocklist December 20, 2025 at 20:00 UTC and December 21, 2025 at 02:35 UTC.
We do not have direct control over this service, however we have been actively working with both Microsoft and Spamhaus to prevent such issues. Despite these ongoing efforts, the block was imposed again.
We are currently communicating with SpamHaus regarding these incidents, but we have not had any success so far.
We recommend that clients resend any affected emails that may not have been delivered during the disruption window.
For the referance, this is an reply from Spamhous regarding this block:
"The below IP is being delisted.
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IP: 52.186.121.110
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However, in the same subnet, there are a significant number of spamming IPs and this has a detrimental effect on any other IPs nearby.
The issue in this case is the neighbourhood and the real solution is for
the provider to restrict the provisioning of their cloud resources to bad actors for short intervals.
While this IP is being removed, we cannot rule out a repeat due to this
egregious behaviour. You may wish to ask your cloud provider how they
plan to combat this persistent abuse of resources."
Recommended Solution: Switch to the Xink 365 Add-in
To avoid similar disruptions in the future, we strongly recommend switching to the Xink 365 Add-in, which does not rely on email routing or Spamhaus. With the add-in:
✅ No reliance on email servers – signatures are applied directly in Outlook before sending.
✅ No risk of email filtering issues – no interaction with external spam blacklists.
✅ Instant updates – signatures are applied automatically without waiting for mail flow rules.
Most of our clients have already switched and eliminated email delivery risks altogether.
Our team can assist with the transition to ensure a seamless experience.
We strongly encourage you to consider the 365 Add-in for a more reliable and future-proof email signature solution.
Dec 21, 09:52 CET